This invention relates generally to the field of permanent magnet motors and more particularly to such motors which sense rotor position using back EMF.
According to Faraday's law when a time-varying magnetic flux "phi" interacts with a coil of wire having "N" turns, an electromotive force (EMF or voltage) is produced which is equal to the time rate of change of the magnetic flux times the number of turns. The polarity of this induced voltage tends to oppose current flow in an energized motor winding: hence the term "back" EMF.
In a permanent magnet motor, the source of the magnetic flux "phi" is a set of permanent magnets mounted on the rotor of the motor. The faster the motor turns, the faster the magnetic field changes relative to the stator windings of the motor, thus creating a larger back EMF. This back EMF can be used to provide position information of the rotor with respect to the stator in order to allow brushless commutation of the permanent magnet motor.
Numerous systems in the past have used back EMF commutation techniques. Early systems of this type were primarily unipolar and linked the back EMF directly or semi-directly to the switching devices so that commutation of the various phases of the motor was a direct result of the back EMF measured for the various phases. More complex systems have subsequently been designed, with a correspondingly high parts count. For example, some systems integrate the back EMF signal to obtain position information.
Still other systems have used passive filters to determine the commutation point after a zero crossing of the back EMF is sensed. These latter systems do have the advantage of being simple and requiring low-cost hardware, but their advance angle is not adjustable, but rather varies inherently with speed.
Back EMF systems heretofore have also suffered from noise in the back EMF signal. This noise adds error to the position signal generated by these systems, which error can be objectionable.
In addition to the back EMF systems outlined above, other brushless commutation systems have used Hall or optical sensors for position feedback. Such sensors, however, add hardware cost and occasionally suffer from reliability problems due to the harsh environment in which the sensors must often operate.
U.S. Pat. No. 4,743,815 to Gee et al addresses many of the above concerns, yet even the system shown in Gee et al could be improved for certain applications. In Gee et al the zero crossing of the back EMF of each phase is sensed to provide position information. In certain high torque or high current conditions, the system of Gee et al could suffer loss of position information because the zero crossing of the open phase was blanked out (i.e., not detected) by the system during commutation.